Synchronizing signal separator



y 26, 1953 D. R. CLEMENTS 2,640,103

SYNCHRONIZING sxcmu. SEPARATOR Filed Feb. 13, 1952 FRAME 7' /ME C4 VIDEO (7 //VPUT NEG. BIAS Inventor DRCLEMENTS By ttorney Patented May 26, 1953 SYN CHRONIZING SIGNAL SEPARATOR Desmond Reginald Clements, London, England, assignor to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application February 13, 1952, Serial N 0. 271,362 In Great Britain February 19, 1951 1 Claim. 1

This invention relates to television receivers and in particular to separating circuits for deriving line and frame synchronising pulses for controlling the scanning of the screen of the cathode ray tube.

Known separating circuits which are capable of producing steep-fronted pulses suffer from the disadvantage that these pulses are invariably of negative polarity which prevents their direct application to certain types of time bases e. g. those employing gas-filled relays of the thyratron type.

The present invention provides a separating circuit which is capable of producing steepfronted positive pulses which may be applied directly to such time base circuits.

An embodiment of the invention will now be described with reference to the accompanying schematic drawing.

Mixed picture and synchronising signals are applied via a resistance-capacity network RLCI to the control grid of a pentode value VI. It is assumed that in these signals, the picture signals are negative-going and the synchronising pulses positive-going. This is in accordance with American practice but is the reverse of standard British practice. In the latter case it is assumed that some signal inversion process has been carried out prior to the stage shown in the drawing.

The time constant of the input network RI .CI is such that the valve is cut off by black level and picture signals while the line and frame pulses appear with negative polarity at the screen grid. The suppressor grid is connected via a resistor R5 to a negative potential large enough to cut 011 the anode current. The anode and screen grid are connected to the positive H. T. supply via appropriate dropping resistors R6 and R1 respectively.

The separated pulses appearing at the screen grid of valve VI are applied via a resistancecapacity network C2.R2 to the control grid of a second valve V2. This second valve is in this embodiment a pentode having its anode and screen grid connected to the positive H. T. supply via appropriate dropping resistors R8 and R9 respectively and having its suppressor grid at cathode (earth) potential. The valve VZ acts as a phase-changing device to convert the pulses to positive polarity. Line synchronising pulsesmay be conveniently derived from the screen grid of valve V2 as shown or, if desired, from the anode.

The frame and line pulses appearing at the anode of valve V2 (which are now substantially in phase with the pulses applied to the control rid of valve VI) are fed back via a resistancecapacity network R3.C3 and a decoupling capacitor C5 to the suppressor grid of valve VI. The network R3 .03 forms an integrating circuit, the time-constant of which is such as to give a large difference between the voltages produced respectively at the suppressor grid of valve VI by line pulses and frame pulses. In other words, the voltages applied to the suppressor grid of valve VI are determined by the length of the pulses and the longer frame pulses therefore produce the greater potential buildup. The negative bias on the suppressor grid of valve VI is of such magnitude that an integrated line pulse does not drive the suppressor grid sufficiently positive to allow anode current to flow. 0n the other hand, an integrated frame pulse drives the suppressor grid above its cut-off voltage and allows the anode to draw current and thus become more negative.

At the end of the first frame pulse the valve VI is cut off at the control grid and a steep positive pulse is produced at the anode which is taken via the resistance-capacity network C434, to synchronise the frame time base.

While the principles of the invention have been described above in connection with specific embodiments and particular modifications thereof, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the invention.

What I claim is:

A circuit for separating line and frame synchronising pulses in a television receiver comprising means for applying all the said pulses to one input circuit of a pentode electric discharge device, means for feeding an output from said pentode to the input circuit of a second electric discharge device, means for applying the output from said second device to another input circuit of said pentode through an integrating circuit such that the combination of a frame pulse and an integrated frame pulse produces a second output from said pentode, means for deriving line synchronising pulses from an output of said second device and means for deriving frame synchronising pulses from an output of said pentode.

DESMOND REGINALD CLEMEN'I'S'.

No references cited. 

